Just to make extra sure that all is in place:
- the physical directory is NOT created on the server
- Your assets/ (assets/Uploads) directory has sufficient rights
- user/group in the assets/ directory includes the webserver!
- safe_mode is off
- open_basedir is none, or at least includes the path
- You've checked you phpinfo for safe-mode/open_basedir, just in case
That's all I can think of for now - I don't use plesk and I can't reproduce. Maybe someone else on this forum? Let me know if you find out...